(a) Field of the invention:
The present invention relates to an optical read-out lens system for optically recorded disks.
(b) Description of the prior art:
For an optical read-out lens system for optically recorded disks such as video disks, it is necessary to make the size of light spot on the disk 1.mu. or less because the lens system is used for reading out the very small signals recorded with high density on the disk. Besides, a reasonably large field angle is required in order to obtain a favourable light spot by continuously focusing the light onto the recorded portion on the optically recorded disk, which rotates at high speed, even when there is something imperfect in an automatic focusing mechanism or when there is inclination of a read-out lens caused due to tolerance in mounting accuracy of the read-out lens. Moreover, in some tracking methods, it is so arranged that the tracking signal is to be obtained through the lens system and, therefore, a reasonably large field angle is required in order to obtain the tracking signal. Furthermore, if the optically read-out lens for optically recorded disks comes into contact with the optically recorded disk, the optically recorded disk and read-out lens are broken. To prevent the above, the optical read-out lens system for optically recorded disks should have a long working distance. Besides, monochromatic light is generally used as the light to be supplied to the read-out lens system. Therefore, to increase the signal-to-noise ratio of playback signals amplified after being detected by a light detector, the optical read-out lens system for optically recorded disks should be made as transparent as possible for the light of the wavelength to be used. To increase the transparency of the lens system, the number of lenses constituting the optical read-out lens system for optically recorded disks should be limited to the minimum required number and lens surfaces thereof should be provided with antireflection coatings which ensure sufficient effect.
Recently, semiconductor lasers are being used as light sources for playback devices for optically recorded disks. To form a light spot of 1.mu. or less by the light of semiconductor laser with the wavelength about 800 nm, the read-out lens system should have large N.A. about 1.3 times compared with the case that He-Ne laser is used as the light source.
Besides, the playback devices employing semiconductor laser are now being put to practical use. Therefore, compared with the experimental stage up to the present, it is now required to provide an optical read-out lens system for optically recorded disks having still longer working distance.